The term oxymethylene polymer as used herein is meant to include oxymethylene homopolymers and diethers and diesters. Also included are oxymethylene copolymers, which include oxymethylene polymers having at least 60 percent recurring oxymethylene units and at least one other unit derived from a monomer copolymerizable with the source of the oxymethylene units.
Oxymethylene polymers having recurring --CH.sub.2 O-- units have been known for many years. They may be prepared for example, by the polymerization of anhydrous formaldehyde or by the polymerization of trioxane, which is a cyclic trimer of formaldehyde, and will vary in physical properties such as thermal stability, molecular weight, molding characteristics, color and the like depending, in part, upon their method of preparation, on the catalytic polymerization technique employed and upon the various types of comonomers which may be incorporated into the polymer.
While the high molecular weight oxymethylene polymers are relatively thermally stable, various treatments have been proposed to increase the polymers utility by increasing thermal stability. Among these are end capping of hemiformal groups of polyoxymethylene homopolymers and hydrolysis to remove unstable groups of oxymethylene in copolymers containing interspersed stable units, such as ethoxy groups. Even beyond these treatments, it has been found necessary to incorporate various stabilizers, antioxidants and chain-scission inhibitors into the polymers.
Unfortunately, oxymethylene polymers are subject to degradation, particularly under the influence of heat. The degradation results mainly from the following three processes:
1. Thermal degradation of the chain end with liberation of gaseous formaldehyde. This degradation which takes place largely under the influence of heat, is often obviated by the presence of either an ether or an ester group at the end of the polymer chain.
2. Oxidative attack leading to chain scission and depolymerization. This is often retarded by the addition of antioxidants to the composition such as compounds containing phenolic or amino groups.
3. Acidolytic cleavage of the chain may occur which also liberates formaldehyde. Acidolytic degradation arises from the presence of acidic species originating from one of several sources: (A) acidic catalyst residues which may have been used in preparation of the polymer, (B) formic acid formed in situ when the trace quantities of formaldehyde generated in processing are oxidized, and (C) acetic acid generated from acetate end groups when a given chain, so stabilized, depolymerizes as a result of occasional oxidative or acidolytic chain scission. To alleviate this condition and to prevent degradation of the polyoxymethylene copolymer especially during subsequent processing in the hot state, "formaldehyde acceptors" or "acid scavengers" are often admixed with the polymer composition. Among the compounds which can be used for this purpose are hydrazines and their derivatives, ureas, certain amides and diamides, polyamides, and metallic salts of acetic acid and fatty acids.
Among the most successful and widely used thermal stabilizers are nitrogen containing compounds which function as formaldehyde acceptors and acid scavengers. These have been found to be effective in lowering the thermal degradation rate of the polymer.
Among the preferred nitrogen compounds used to thermally stabilize oxymethylene polymers are amidine compounds, i.e., compounds having a carbon atom doubly bonded to one nitrogen atom and singly bonded to another. An especially preferred class of amidine compounds are the N-substituted amidine compounds wherein another nitrogen atom is singly bonded to the amidino group, most preferably at the carbon atom.
Suitable amidine compounds include the cyano-guanidine compounds including cyanoguanidine, itself, and other compounds including those containing the divalent 1-cyano-3,3 guanidino radical: ##STR2## Among the substituted cyanoguanidines which may be used are those having one or two suitable inert substituents at the 3-nitrogen position of the guanidine nucleus. For example, in the above formula, R.sub.c and R.sub.d may be the same or different inert substituents including hydrogen, alkyl, aryl, cycloalkyl, hydroxylalkyl, haloalkyl, haloaryl and other substituents. Specific compounds which are suitable include 1-cyano-3 methyl guanidine, 1-cyano-3 ethyl guanidine, 1-cyano-3 isopropyl guanidine, 1-cyano-3,3-diphenyl guanidine, 1-cyano-3-hydroxymethyl guanidine, 1-cyano-3-dodecyl guanidine, 1-cyano-3-(2-hydroxyethyl) guanidine, 1-cyano-3-(2-bromoethyl) guanidine 1-cyano-3-(m-chlorophenyl) guanidine and 1,3-dicyanoguanidine.
Another useful stabilizer system disclosed in the prior art contains a cyclic amidine compound, e.g., having a ring carbon doubly bonded to one ring nitrogen atom and singly bonded to another ring nitrogen atom. Preferably the cyclic amidine compounds are devoid of carbon-to-carbon ethylenic unsaturation.
The preferred compounds of this class are amine-substituted derivatives of symmetrical triazines, including guanamines (2,4-diamino sym. triazines), melamine (2,4,6-triamino sym. triazine) and substituted melamines. The amino groups may be primary, secondary or tertiary and other substituents such as hydroxyl substituents may be present. Of course, the amino groups and other substituents must be those which are inert, i.e., will not induce undesirable reactions. Among the specific compounds which are suitable are 2,4-diamino-6-phenyl sym. triazine (benzoguanamine); 2,4-diamino-6-methyl sym. triazine; 2,4-diamino-6-butyl sym. triazine; 2,4-diamino-6-benzyloxy sym. triazine; 2,4-diamino-6-butoxy sym. triazine; 2,4-diamino-6-cyclohexyloxy sym. triazine; 2,4-diamino-6-chloro sym. triazine; 2,4-diamino-6-mercapto sym. triazine; 2,4-dihydroxy-6-amino sym. triazine (ammelide); 2-hydroxy 4,6-diamino sym. triazine (ammeline); N,N,N',N'-tetracyanoethyl benzoguanamine; 2,4,6-triamino sym. triazine (melamine); phenyl melamine; butyl melamine; N,N-diethyl melamine; N,N-di-(2-hydroxyethyl melamine; N,N-diphenyl melamine; N,N-diallyl melamine; N,N',N"-trimethyl melamine; N,N',N"-triethyl melamine; N,N',N"-tri(n-propyl) melamine; N,N',N"-tri(n-butyl) melamine; N,N,N',N"-tetramethyl melamine; trimethylol melamine; trimethoxymethyl melamine; hexamethoxymethyl melamine; N,N',N"-triphenyl melamine; and N,N',N"-trimethylol melamine.
Still another preferred stabilizer is a compound containing both amine and amide groups and having from 0.2 to 5 amide groups per amino nitrogen atom ("amine-amides"). Preferred amine-amides are compounds wherein the amine groups are tertiary amine groups and wherein there are from one to three amide groups per amino nitrogen atom. Suitable amine-amides include compounds containing the structure N-R.sub.a -Z-R.sub.b wherein R.sub.a is a divalent organic radical having terminal carbon bonds, Z is a ##STR3## group which may be in any position with respect to the other atoms of the molecule, and R.sub.b is hydrogen or a monovalent organic radical having a terminal carbon bond, e.g., an alkyl or aryl group either unsubstituted or containing only inert substituents. The preferred compounds are compounds in which R.sub.b is hydrogen and the free nitrogen bonds are singly linked to carbon atoms; such compounds have terminal amide groups, i.e., ##STR4## groups and tertiary amino groups. The free nitrogen bonds at the left of the formula in the preferred compounds may be linked to alkyl groups, aryl groups or may be linked through carbon atoms to the other atoms of a heterocyclic ring. The free nitrogen bonds may be linked to additional radicals having amide groups. If desired, the radical linked to the free nitrogen bonds may have one or more additional tertiary amino nitrogen atoms in their skeletons or may even comprise a repeating polymeric structure.
The divalent radical R.sub.a may be an alkylene radical, such as a methylene, ethylene or butylene radical, or an arylene radical, such as a phenylene radical. These radicals may be unsubstituted or may contain substituents which are inert. The divalent radicals may also, if desired, have one or more additional tertiary nitrogen atoms in their skeletons.
Among the specific tertiary amine-amides disclosed to be suitable are nitrilo-tris-beta propionamide; beta (4-morpholinyl) propionamide; N,N-diemthyl-p-carbamyl aniline; 4-diethylamino-2-methyl acetanilide and p-diethylamino acetanilide.
While the above defined stabilizers have been found useful in improving the thermal stablization of oxymethylene polymers, used alone, these stabilizers present problems which make these stabilizers somewhat undesirable for commercial applications. Foremost is the inability of these mentioned stabilizers to reduce formaldehyde levels in the polymer. Thus, while the N-containing stabilizers are weak bases which function to neutralize acids and thus reduce acidolytic cleavage of the oxymethyl polymer chain and readily react with formaldehyde to function initially as formaldehyde acceptors, the compounds do not hold onto the formaldehyde as the reverse reaction to release formaldehyde is as strong as the reaction to accept formaldehyde. Consequently, the level of free formaldehyde present in the molded components reduces the commercial applicability of these stabilizers for obvious environmental reasons.
A particularly preferred stabilizer and one that has found use in commercial applications is characterized as a superpolyamide. The superpolyamide stabilizers are the macromolecular superpolyamides, commonly known as nylons, that have carboxamide linkages ##STR5## as integral portions of their polymer chains. These superpolyamides preferably have melting points below approximately 220.degree. C., in which R represents a hydrogen atom, a lower alkyl group, or a lower alkoxy group, and have a degree of polymerization of approximately 100 to 200. A preferred group of the superpolyamides includes those condensation polymers that on hydrolysis yield either omega-aminocarboxylic acids or mixtures of dicarboxylic acids and diamines.
The superpolyamide stabilizers are most useful because, although less active as a formaldehyde acceptor than certain of the triazines, the superpolyamides hold onto the formaldehyde as the reverse reaction to release formaldehyde is slow. Consequently, oxymethylene compositions employing a superpolyamide stabilizer contain lower levels of extractable formaldehyde than compositions employing the other mentioned N-containing stabilizers. However, there still exists the need for improvement. Oxymethylene polymer compositions employing a superpolyamide stabilizer have a tendency to discolor under thermal stress. Recently it has been found that such superpolyamide polymers form mold deposits upon molding under high shear conditions. The present invention provides improvements to oxymethylene polymer compositions employing superpolyamide stabilizers.